1. Field of the Invention
The present invention relates to apparatus for gas assisted thermoplastic inaction molding. More particularly, the present invention relates to a flow front control system therefor which provides for selective control of termination of plastic inaction and introduction of pressurized gas based upon predetermined processing of sensed flow front location information from inside the mold.
2. Description of the Prior Art
Gas assisted plastic inaction molding is a well established and commercially accepted method for providing plastic articles having an interior hollow. Such hollow plastic articles have numerous advantages, such as high strength, low weight, improved article appearance due to less shrinkage stress, and reduced plastic material cost A detailed discussion of the development of gas assisted injection molding technology is contained in U.S. Pat. No. 5,110,533.
Gas assisted plastic injection molding typically involves a mold having an interior cavity which defines the exterior shape of the plastic article to be provided, a sprue having an inaction aperture and a gate into the cavity, a plastic inaction apparatus which connects to the inaction aperture and rams a shot of thermoplastic thereinto, and a pressurized gas source which introduces pressurized gas into the injection aperture of the sprue. In typical operation, a shot of plastic is injected into the cavity via the sprue, then pressurized gas is introduced which causes the plastic to expand against the cavity walls. The gas pressure is maintained until the plastic has cooled sufficiently to be self supporting, whereupon the gas is vented, the mold opened, and the plastic article is removed from the cavity.
An example of gas assisted injection molding technology is presented in U.S. Pat. No. 5,118,455. Herein, a thermocouple is positioned to sense the temperature of the plastic resin at the injection aperture, and another thermocouple is positioned to sense the temperature of the plastic resin in the cavity. A control panel interprets the thermocouple signals and thereupon regulates the gas pressure to suit the viscosity of the plastic resin.
Problematically, the timing of the introduction of the pressurized gas is critical. If too early, the pressurized gas can blow past the flow front of the plastic thereby ruining the article, and if too late, then various objctionable stress marks can appear on the surface of the article, again thereby ruining the article.
A gas assisted plastic injection molding technology which is aimed at solving the problem of when to introduce the pressurized gas is described in U.S. Pat. No. 5,149,547. In this technology, introduction of the pressurized gas is based upon measurement of the amount of plastic shot that has been delivered to the cavity. An ideal amount of shot is first determined and the amount of plastic delivered per spindle revolution of the injection apparatus is also determined. The measurement of the shot is then provided by counting the revolutions of the spindle and then comparing this to a predetermined number of revolutions which would represent the desired shot.
A gas assisted plastic injection molding technology which is aimed at controlling the pressurized gas delivery and venting is described in U.S. Pat. No. 5,198,238. In this technology, a temperature or pressure sensor is located in the cavity of the mold and a microprocessor then uses sensed data therefrom to selectively control the operation of gas entry and gas venting.
While shot volume measurement is adequate in principle, in practice it is subject to the vagaries of temperature and pressure variations which may make the shot volume determination imprecise. Also, while microprocessor control over the pressurized gas is a laudable advance in the art, there remains needed not only microprocessor control over the pressurized gas but also the cessation of plastic injection. Accordingly, what remains needed in the art of gas assisted plastic injection molding which is a precise and reproducible indication of when the plastic injection is to cease and when the pressurized gas is to be introduced.